Exhaust gas turbochargers are known to be used for increasing the power of an internal combustion engine. In such an exhaust gas turbocharger, a turbine is provided in the exhaust gas path of the internal combustion engine, and a compressor is arranged upstream of the internal combustion engine, which is connected to the turbine via a common shaft. With charging of an internal combustion engine by means of an exhaust gas turbocharger, the capacity and therefore the fuel mixture in the cylinders are increased and a noticeable power increase for the engine is thereby gained. Optionally, the energy which is stored within the exhaust gas of an internal combustion engine can be converted into electrical or mechanical energy by means of a power turbine. In this case, instead of a compressor, as in the case of the exhaust gas turbocharger, a generator or a mechanical consumer is connected to the turbine shaft.
An exhaust gas turbocharger is generally constituted by a rotor, a bearing assembly for the shaft, flow-guiding housing sections (compressor housing and turbine housing) and a bearing housing. The rotor includes a shaft, an impeller and a turbine wheel.
Because of the high process pressure in the turbine-side and in the compressor-side flow region, the shaft of the exhaust gas turbocharger is sealed with a suitable sealing concept in relation to the cavity of the bearing housing. The internal pressure in the cavity of the bearing housing usually corresponds to the atmospheric pressure. The gas pressure in the flow passage of the compressor side and turbine side depends, however, upon the current operating point of the exhaust gas turbocharger and at most operating points lies above the pressure in the cavity of the bearing housing. In certain cases, however, a negative pressure is also to be taken into consideration, for example, in partial load operation or at rest.
DE 20 25 125 discloses a turbine-side shaft seal of an exhaust gas turbocharger, which consists of a simple oil-collecting chamber on the turbine side of the radial bearing, and a piston ring with a sealing effect between the shaft and the bearing housing. The bearing oil which issues from the radial bearing splashes onto the outwardly offset and rotating shaft shoulder and, as a result of centrifugal forces, is thrown into the oil-collecting chamber. The bearing oil which is thrown out in this way then flows downwards inside the oil-collecting chamber as a result of the force of gravity and flows back again into the oil circuit of the bearing lubricating system.
For reducing gas leakage from the flow passage through the wheel back space of the turbine into the cavity of the bearing housing, piston rings made of metal, for example, gray cast iron, are generally used. The piston ring under tension is clamped in a radial groove with an axial stop shoulder in the bearing housing. As a counterpart to the piston ring, the rotating shaft is provided with a radial groove, wherein the piston ring is axially trapped inside this groove and radially overlaps the groove. Because of the pressure difference between the exhaust gas pressure and the pressure inside the bearing housing, the piston ring is axially displaced as far as it will go in the direction of the existing pressure gradient inside the groove. As a result of the axial seating of the piston ring on one of the inner surfaces of the groove, this piston ring grinds itself in and seals the bearing housing plenum relative to the exhaust gas flow. For improving the sealing effect, two or more piston rings can also be used, as is disclosed in CH 661 964 A5, U.S. Pat. Nos. 3,180,568, 4,196,910 or EP 1 860 299, for example. In these documents, it is shown how the sealing effect against the hot exhaust gases can be increased by means of the additional use of sealing air or deaeration of the space between the two piston rings, and consequently how escape of the exhaust gases into the bearing housing can be totally prevented.
DE 37 37 932 A1 discloses a turbine-side shaft seal of an exhaust gas turbocharger, in which the oil outlet from the radial bearing is provided between the bearing point and the two piston rings. In this case, for improving the oil tightness, an additional centrifugal oil slinger is used instead of a simple axial shaft shoulder. The amount of impinging unwanted bearing oil in the region of the piston ring groove can be significantly reduced as a result. Similarly, in the shaft seals according to U.S. Pat. No. 4,268,229 and also DE 30 21 349, the oil outlet is provided between the radial bearing and the adjacent piston ring, wherein the oil outlet still consists of a chamber. In addition, the cavity between the two piston rings is connected by means of an additional connecting passage to the cavity of the bearing housing and aerated to atmospheric ambient pressure. The resulting pressure difference across the left-hand piston ring is consequently prevented so that the piston ring predominantly undertakes an oil-sealing but not hot-gas sealing function. Therefore, only the right-hand piston ring undertakes the sealing between the pressurized flow passage and the cavity of the bearing housing. As a result of these construction variants, two separate outlets are therefore created for the media of oil (from the radial bearing) and also exhaust gas (from the flow passage), wherein the outlets are separated by means of a piston ring. The lubricating oil which issues from the radial bearing possibly splashes axially into the piston ring region of the gas seal and in the most unfavorable case floods the entire piston ring groove. The gas pressure in the flow passage of a compressor or a turbine is generally greater than the internal pressure in the bearing housing of the turbocharger. Thus, a positive pressure difference (pressure in the flow passage is higher than in the cavity of the bearing housing) results in the ensuing gas leakage blowing through the piston ring seal, and the bearing oil which has inadvertently penetrated into the piston ring region is carried back into the oil-collecting chamber of the bearing housing.
DE 10 2004 055 429 B3 discloses a sealing device for a lubricated bearing of a rotor shaft, which seals a bearing housing of a turbocharger against a supplied lubricating oil in the axial direction. Provision is made on the rotor shaft for a first seal in the form of a gap, a labyrinth or a piston ring, and for a second seal in the form of a narrow gap or a labyrinth, which between them include an oil outlet passage which extends annularly around the circumference of the rotor shaft and is constructed by means of a housing-side oil outlet groove and a shaft-side oil outlet groove which is arranged in an axially aligned position. Provision is made in the oil outlet passage for an annular sealing web which, in the radial direction of the rotor shaft, projects by one end freely into the annular oil outlet passage, and which constitutes a barrier acting in the axial direction for the lubricant which penetrates into the oil outlet passage and radially overlaps the gap of the second seal.
DE 43 30 380 A1 discloses an exhaust gas turbocharger which includes a two-section bearing housing in which oil for cooling is splashed from a first section onto the surface of the second section.
In the case of the above-described turbine-side shaft seal concepts, under certain circumstances there is the risk that hot gases from the wheel back space of the exhaust gas turbine escape through the piston ring seal, and that the bearing oil which remains in the piston ring region and also in the oil outlet grooves locally burns and consequently creates serious coking of the shaft seal and wear associated therewith. The risk of coking increases with rising exhaust gas temperature and with increased gas leakage through the piston rings and also with poorer component cooling. Thus, active cooling of this sealing section is desired for the operational reliability of the shaft seal.
DE 197 13 415 A1 discloses an exhaust gas turbocharger which has, in the region of a thrust bearing at the rear of the impeller, an annular sealing plate as an oil splash guard.
US2005/0188694 discloses an exhaust gas turbocharger which has, between two piston rings in the region of the shaft seal at the rear of the impeller, an oil suction pipe by means of which the zone between the two piston rings is cleaned of possibly penetrating oil by means of a vacuum pump.
U.S. Pat. No. 4,523,763 discloses an exhaust gas turbocharger which, in the region of the shaft seal at the rear of the impeller, has a labyrinth seal to prevent oil from the lubrication circuit being able to reach the operating chamber of the compressor.